Abstract: Ab initio calculations have been carried out at the Hartree-Fock/6-31G^*level to determine the geometries of the nereistoxin(C₅H₁₁NS₂)and evisect(C₅H₁₁NS₃).Two stable conformations of chair and boat forms of the evisect have been found in the global optimizations.At the second Moller-Plesset perturbation theory,MP₂/6-31G^* level, the chair form is 27.06 kJ/mol which is more stable than the boat form.The molecular electrostatic potentials obtained from MP₂/6-31G^* wave-functions.The relationship between biological activity and electrostatic potentials have been discussed.The atomic charges from Mulliken population analysis is faulty in the nereistoxin and evisect.Brenneman's CHELPGmethod was used in the calculations.

Key words: Nereistoxin, Evisect, Ab initio, Geometry, Molecular electrostatic potential